1. Field of the Invention
This invention relates generally to weighted surface acoustic wave reflectors for use in a SAW filter or SAW resonator and, more particularly, to weighted surface acoustic wave reflectors that employ grid lines having randomly distributed acoustic reflective dots within gaps between adjacent grid lines, on the grid lines or on or between non-periodically spaced grid lines to provide the desired net reflectivity.
2. Discussion of the Related Art
Surface acoustic wave (SAW) filters for use in mobile phone communications systems are designed to be small in size, exhibit good out-of-band rejection, and provide narrow bandwidths with steep transition edges. Conventional SAW filters include an input transducer and an output transducer formed on a piezoelectric substrate. The input transducer is electrically excited with the electrical input signal that is to be filtered. The input transducer converts the electrical input signal to surface acoustic waves, such as Rayleigh waves, lamb waves, etc., that propagate along the substrate to the output transducer. The output transducer converts the acoustic waves to a filtered electrical signal.
The input and output transducers typically include interdigital electrodes formed on the top surface of the substrate. The shape and spacing of the electrodes determines the center frequency and the band shape of the acoustic waves produced by the input transducer. Generally, the smaller the width of the electrodes, or the number of electrodes per wavelength, the higher the operating frequency. The amplitude of the surface acoustic waves at a particular frequency is determined by the constructive interference of the acoustic waves generated by the transducers.
The combined length of the transducers determines the length of the overall filter. To design a conventional SAW filter with ideal filter characteristics, the filter's impulse response needs to be very long. Because the length of the impulse response is directly proportional to the length of the transducer, the overall length of a conventional SAW filter having ideal characteristics would be too long to be useful in mobile phone communications systems.
Reflective SAW filters have been developed to satisfy this problem. Reflective SAW filters generally have at least one input transducer, one output transducer and one reflector formed on a piezoelectric substrate. The reflector is typically a reflective grating including spaced apart grid lines defining gaps therebetween. The acoustic waves received by the reflector from the input transducer are reflected by the grid lines within the grating so that the reflected waves constructively and destructively interfere with each other and the wave path is folded. Because of the folding, the length of the transducer is no longer dependent on the duration of the impulse response. Reflective SAW filters are, therefore, smaller in size and have high frequency selectivity, and thus are desirable for mobile phone communication systems.
The frequency response of a reflective SAW filter is further improved by weighting the individual reflectors to achieve a desired net reflectivity. Existing weighting methods include position-weighting, omission-weighting, and strip-width weighting. Other methods of weighting reflectors include changing the lengths of open-circuited reflective strips within an open-short reflector structure. Weighting the reflector improves the frequency response by reducing passband ripple and reducing sidelobe levels in the rejection band.
The above methods of weighting a reflector are all dependent upon the critical dimension of the reflector. The critical dimension of a reflector is the smallest reflector grid width or gap width, and is inversely proportional to the operating frequency of the filter. As the operating frequency increases, the critical dimension decreases. Fabrication constraints limit the critical dimension, thus limiting the operating frequency of the filter. As the operating frequency increases, most reflectors will have a limited dynamic range when implementing a wide range of reflectivity, which is required for filters with high selectivity. A reflective filter that is not as tightly constrained as current filters by its critical dimension would be advantageous.
An ideal frequency response for a reflective filter has a high frequency selectivity with steep transition edges, giving the response a good shape factor. If the critical dimension of a reflector were independent of the reflector strength, a wide range of reflectivity could be achieved that would produce a narrow bandwidth and steep transition edges.
What is needed is a reflector having a reflectivity function that is not as limited as today's SAW filters by the critical dimension of the structure, and that is able to operate with high reflectivity and with high frequency selectivity.